As is known, a microchannel plate (MCP) includes an array of small diameter tubes or channels, each of which operates as an independent electron multiplier in the presence of an electric field applied to the MCP. As a signal (e.g., an electron, photon, ion) enters the input end of a given channel and passes through that channel, it impacts the channel walls thereby producing so-called secondary electrons that then also propagate through the channel and impact the channel wall to produce even more secondary electrons. This repetitive addition of electrons effectively amplifies the original input signal by several orders of magnitude, depending on factors such as strength of the electric field and channel geometry.
A collector electrode (generally referred to as an anode) is provided at the other end of the channel to collect the multitude of electrons (sometime referred to as an electron pulse or cloud). While some MCP designs have a single anode to collect total current produced by all channels, other MCP designs have a multi-anode configuration where each channel has a dedicated anode. Such a multi-anode MCP configuration is particularly useful when it is necessary to maintain spatial relationships of input signals (e.g., such as the case with imaging applications).
MCP devices can be used in a number of detectors for military, scientific and commercial applications. In general, a detector that employs MCP technology includes a converter (e.g., photocathode) to convert the incident photons into electrons, one or more MCPs that operate to amplify the initial electron or photon event into an electron cloud, and a readout circuit for receiving each electron cloud and converting it into a signal having qualities suitable for subsequent signal processing. MCPs are in general sensitive to photons by a much lower efficiency than a photocathode. In some cases, however, where the MCP is directly sensitive to the target event or particle, no converter is needed (e.g., such as in ion detection in mass-spectrometry applications, and UV and VUV radiation detection applications). In other cases, the converter may further include a scintillator that converts incident particles into photons that are subsequently converted to electrons by a photocathode or other suitable conversion mechanism.
Current microchannel plates are typically made from doped glass, but can also be made from other materials such as silicon. Regardless of the material used, a problem associated with such conventional MCP-based detectors is that they have limited dynamic range due to the maximum current that can be dissipated in the MCP. Specifically, the dynamic range is effectively set by the limit on the strip current (total current flowing through the device). In typical operation, the MCP channels that have had an event (photoelectron) become depleted of charge, and thus the channels must recharge as to be ready for the next event in the channel. To this end, the MCP is connected to a high voltage bias that recharges the channel through the resistance of the plate. This resistance, however, is selected to keep the current below a thermal runaway condition (generally caused by reduction of plate resistance as the temperature increases). Unfortunately, such a constraint increases the time for the charge to build-up in depleted channels. This charge build-up time effectively defines the minimum time limit between when events can be detected. Hence, the dynamic range of MCP-based detectors is limited, and event information may go undetected.
One conventional solution to extend the dynamic range of a MCP-based detector is referred to as gating, which involves turning the photocathode off for part of the integration period. However, there are a number of issues with this approach. First, the complexity of the hardware is increased as well as a loss of signal even in the 100% on state. Second, the so-called dim signals will be lost when gating is implemented. Third, there is a negative impact on signal processing algorithms due to inaccuracies associated with the gating process, which gives rise to a need for calibration.
There is a need, therefore, for techniques that can be used to increase the dynamic range of an MCP device.